1. Field of the Invention
This invention relates to a prosthetic bearing assembly of the kind comprising an outer fixation element and an inner bearing component. Bearing assemblies of this type are used in various orthopedic prostheses and the present invention is particularly, although not exclusively, applicable for use with prosthetic acetabular cups.
2. Description of the Prior Art
The use of prosthetic implant devices in the human body for replacing defective, damaged or diseased anatomical joints of the body has become quite well known. One of the more common forms of implant devices is the hip joint prosthesis which provides an interconnection between the femur and the acetabular socket of the pelvis. Conventional prosthetic hip joints usually include an artificial femoral component having a spherical head member which is received within a complementary spherical socket provided by an acetabular cup assembly.
One frequently used form of acetabular cup assembly has a metal shell component, which is secured to the acetabulum, and a bearing insert component, usually constructed of plastic, secured within the metal shell component to complete the cup assembly. It has been suggested that the bearing insert component be assembled with the shell component interoperatively; that is, the shell component is seated permanently within the acetabulum and then the bearing insert component is inserted into the shell component by the surgeon, thereby providing the surgeon with an opportunity, during implantation, to assess the cup assembly position and hip joint function using provisional trials and enabling minor adjustments prior to commitment to a final implant position. Thus, the bearing insert component is provided with a bearing face which is angled slightly relative to the lower face of the shell component so that by rotation of the bearing insert component relative to the shell component, the orientation and anteversion of the bearing face can be adjusted after seating of the shell component and prior to permanent assembly of the bearing insert component with the shell component.
Prosthetic acetabular cups are known which use a snap-fit between the shell and the bearing or other methods for preassembly of bearing components of different internal diameters. For example, U.S. Pat. No. 4,695,282 shows an acetabular cup assembly which includes an outer metal fixation shell and a plastic bearing insert which is snapped into position. The arrangement is such that the shell can be repositioned circumferentially to a limited number of positions relative to the metal shell. U.S. Pat. No. 4,676,798 also shows a socket bearing assembly in which the shell can be located in different circumferential positions. A disadvantage with this latter construction is that the number of alternative positions is limited and the construction is expensive to manufacture due to the intricacy of the various components.